The present invention relates to an auto-test device to test the transmission and reception system of a radar. It can be applied especially to radars fitted into automobiles; More generally, it can be applied to radars that need to be produced at low cost.
Several types of radars can be fitted into automobiles. Among these are the known radars of the ACC or Automobile Cruise Control type. A radar of this type is designed to enable the cruising speed of automobiles to be regulated. It detects and locates the closest vehicle located in the path of the carrier vehicle. The information transmitted to the computer of the carrier vehicle makes it possible for example for the computer to take action, through appropriate interfaces, on the control system of the engine so as to regulate the distance between the carrier vehicle and its predecessor. In this case, the driver no longer has to activate the longitudinal controls such as the accelerator, the brakes or the gears. Given the vital importance of managing these controls, an analysis of the overall operational safety of the system shows that it is indispensable to have an efficient auto-test function to quickly detect any variation in the characteristics of the radar.
A speed regulation radar for automobiles may for example work by transmission with frequency shift-keying. The radar then transmits a sequence of pulses at difference frequencies. The exploitation of the resultant phases enables the distance D to be computed according to the following relationship in an exemplary use of two frequencies: ##EQU1## where c, .DELTA.F, .DELTA..phi., respectively represent the velocity of light, the difference in frequency of the waves transmitted and the phase difference of the signals received from one and the same target in relation to each frequency.
In practice, it is possible to use a larger number of frequencies. The computation of the distance is then for example derived from a linear combination of the elementary computations according to the above relationship (1). It can be seen in these cases that the measurement of the distance is inversely proportional to the difference in frequency between two transmitted waves. In the present state of W band microwave technology, i.e. pertaining to the 76 GHz-77 Ghz band, this difference is a variable that is difficult to control. Checking the radar during operation is therefore indispensable in view of the vital importance of the reliability of the information influencing the control of the speed of an automobile.
The methods for carrying out auto-tests on radars generally use circuits for the measurement of the characteristics of microwave transmission and circuits for the injection of calibrated signals into the reception channel. These circuits require microwave functions which, especially in the W band, are costly, difficult to implement and liable, by their presence, to induce a significant lowering of the performance characteristics of the main function. These auto-test methods are therefore not compatible with equipment that has to be produced in large batches and at low cost.